20 June 2016

Cushendun Conglomerate of the Cross Slieve Group, Northern Ireland

Posted by Callan Bentley

Want a geological irony? Here’s one!


You’re looking at a rounded boulder of Cushendun Conglomerate, a Devonian “Old Red Sandstone” unit (Cross Slieve Group) exposed at Cushendun Caves, Northern Ireland, U.K. The irony lies in the repetition of history – a tumbling environment of high water energy, rounding cobbles and boulders and depositing them, in order to make the conglomerate. And now, ~400 million years later, history repeats itself, with the same rock! Note the surrounding cobbles, being tumbled and rounded in a very similar way. Compare and contrast:


I found this boulder on the beach at Cushendun, a charming little town on the eastern coast of Northern Ireland. Just south of town are a series of rocky islets and caves cut into a headland of this conglomerate.


Here’s my field assistant in the cave:


The view out among the sea stacks to the countryside beyond:


If you watch the HBO series Game of Thrones, you may recognize Cushendun Caves as the place where [spoiler alert] the red priestess Melisandre gave birth to a shadow demon thing. Check out the conglomerate on the wall behind her!


That scene was not being reenacted when we visited, to mixed emotions.

Anyhow, we weren’t there to be GoT tourists (though there were plenty of other folks doing exactly that!); we were there for the rocks. During the Devonian, mountain-building shuddered to life in the British Isles. As the mountains rose, they shed sediment in vast quantities, and much of it was deposited along the flanks of that ancient range. There were arkoses aplenty to be seen in the building stones along our walk in from the parking area:





But at the caves themselves, the arkose is joined by a bunch of big boulders of quartzite and related rocks, all very well rounded:


In many places, as above, the clasts don’t touch much – the conglomerate is “matrix supported.” The locals call it puddingstone.

Elsewhere, the big grains touch each other, including along grain boundaries that are flush (apparently planar) or impinging on their neighbors in a concave/convex relationship. This is evidence of pressure solution; the dissolving of portions of these cobbles at the highest-stress areas where they touched. Check out the lower/middle right of this outcrop photo, for instance:


Here are four GigaPans so you can check for these features yourself:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Post-depositional stresses fractured these conglomerates with a set of distinct co-parallel fractures, some of which acted as faults. Can you spot the fault in this next image?


Here’s my favorite outcrop at the Cushendun Caves:


Before I explain, see if you can suss it all out.

Okay, I’m not waiting any longer!

In this photo, there are many examples of flush/impinging grain boundaries (circled in white in the annotated version below), plus some of those flush grain boundaries (black) have been offset along a set of small faults (traced out in yellow):


It’s a neat place to visit. If you find yourself in Cushendun, I can’t think of any reason why you wouldn’t make a point of visiting.